1. Field of the Invention
The present invention relates to a control device and a method for controlling a locking-up hydraulic pressure which is provided to a lock-up clutch disposed in a torque converter of an automatic transmission. Said lock-up clutch is a clutch which mechanically connects and disconnects a pump impeller and a turbine runner of the torque converter.
2. Description of the Prior Art
Japanese Unexamined Patent Publication (Kokai) No. 88372/1988 discloses a control method for locking up hydraulic pressure in an automatic transmission.
According to this method, shift operation is performed while lock-up clutch is on i.e. connected mechanically. In this case, the lock-up solenoid valve is controlled in accordance with a duty ratio signal which is different from a usual one from a decision to begin shift operation has made by a transmission signal to the shift operation is finished. This makes the looking-up hydraulic pressure lower than usual. Accordingly, the lock-up clutch begins slipping, and a shook in shift operation is decreased.
However, applying this known control method for the looking-up hydraulic pressure in an automatic transmission has a following drawback. Namely, the engine revolution may rise abnormally in case if the looking up hydraulic pressure is lower than a predetermined level. This may occur by variation among products. It is not desirable to ruin the engine by over-revolution nor to consume fuel too much in vain. On the other hand, in case of setting the-locking up hydraulic pressure in higher level previously to avoid over-revolution, it is not effective in decreasing a shock in shift operation. Consequently, it is very hard to tune the duty ratio on which the looking up hydraulic pressure is determined.